The Queen will celebrate her 96th birthday on Thursday in Sandringham.
Britain's longest-lived monarch travelled by helicopter to her Norfolk estate, where she will be joined by family and friends.
She is expected to stay in a cottage on the estate particularly liked by her late husband, Prince Philip.
A photograph has been released to mark the birthday, showing the Queen with two ponies and reflecting her lifelong interest in horses.
The picture was taken at Windsor Castle, where the Queen now mostly stays, and shows her with two Fell ponies, who will appear in the forthcoming Royal Windsor Horse Show.
Birthday wishes
Prince Harry suggested in a US TV interview that the Queen might be downplaying this latest milestone, saying that "after a certain age you get bored of birthdays".
But she will be spending time at Wood Farm, a cottage described as "small and intimate" by former housekeeper Teresa Thompson, and a place with strong associations with Prince Philip, who died a little over a year ago.
The Queen was in Sandringham earlier this year when she marked her accession to the throne in 1952.
She was born in 1926, not in a royal residence, but in a London townhouse on Bruton Street, where she first lived with her father and mother, who became King George VI and Queen Elizabeth.
Birthday wishes have been sent by Prime Minister Boris Johnson and Labour leader Sir Keir Starmer and gun salutes will be fired.
The Queen has had mobility problems in recent months and has had to miss a number of events, including church services over Easter.
Her only public appearance this year, outside of her own residences or on video, was at the Thanksgiving Service for Prince Philip at Westminster Abbey.
The Queen's 96th birthday marks another unprecedented age for a British monarch, in a year in which she has become the first monarch to celebrate a Platinum Jubilee of 70 years on the throne.
The next longest-lived monarchs were Queen Victoria and George III, who lived to the age of 81, and were the only other monarchs to have lived into their 80s.
According to figures from the Office for National Statistics, the Queen is one of about 124,000 people in the 95 to 99 age group in the UK, almost three quarters of these being women.
A Barbie doll celebrating the legacy of the Queen is being released to commemorate her Platinum Jubilee, featuring a recreation of her wedding tiara and an ivory gown fitted with a blue ribbon.
